1. Field of the Invention
The present field of the invention relates to membrane switches, and more particularly to a method for manufacturing membrane switches that are illuminated using electroluminescent lamps.
2. Description of the Prior Art
Present membrane switches are typically made from flexible plastic insulators that contain two layers of opposing electrically conductive surfaces isolated from one another by an air gap such that, when one surface is mechanically deformed by applied pressure, that deformed surface makes mechanical contact against the opposing stationary surface and completes an electrical current path between them. This current path may carry either signal or power electrical charge, or both. By positioning an insulating mask between these two surfaces, effective mechanical isolation ensures that unwanted electrical contact is avoided. Adding illumination to such membrane switches can create both complicated and bulky assemblies that are unsuitable for many electronics product applications. Illuminated membrane switch assemblies made using this method contain three or more individual layers of electrically conductive and isolating materials that require precise alignment for their successful application.
An alternative construction consists of a rigid circuit board having on its upper surface a pair of electrical switch contacts. Positioned above this surface is an isolating mask layer that is typically a plastic film with openings positioned in alignment with the contact pairs. Above that is placed a second plastic film with a deformable electrical shunt surface oppositely positioned in alignment with the isolation mask""s openings and the printed circuit board""s switch contact pairs. When this outermost shunt layer is mechanically deformed by pressure, the shunt is driven past the isolating mask layer opening such that the shunt may then make contact to the printed circuit board""s switch contacts, thus creating a current path. Illuminating this switch construction may take the form of an overlaying elastomeric actuating structure that is edge-lit illuminated by externally mounted lamps or alternatively via light emitting diodes (LED""s). Application of an additional layer of electroluminescent lamp construction may also be used to provide illumination to the elastomeric structure. Such constructions typically require an additional rigid framework to keep the various layers in alignment.
An alternative to this second construction is to form the elastomeric actuating structure into an integrated system that begins with a positioning flange that rests on top of the printed circuit board and surrounds the switch contact pair. Projecting from this flange structure is an elastomeric spring member that then supports an actuating key. In the open gap formed by this structure, a typically cylindrical shaped protrusion extends down from the actuating key and is supported above the switch contacts. The end of this protrusion may alternatively be coated with a conductive surface to provide the electrical shunting effect, or a xe2x80x9cpillxe2x80x9d of conductive elastomer is attached to the protrusion to provide this function. Thus, the actuating key may be pressed, allowing the shunting surface of the protruding conductor to mechanically contact the switch contacts below to from an electrical current path between them. If an additional insulating layer, constructed with electroluminescent lamp elements that surround an opening in the insulation corresponding to the location of the shunting protrusion of the elastomeric actuating structure, is placed between the elastomeric actuating structure and the surface of the switch bearing side of a printed circuit board, a ring of illumination surrounds the actuating key. Additionally, a rigid framework must also be provided to keep the surfaces and structures in alignment.
In the above alternative methods, only signal level electrical charge may be switched by key actuation. Additionally, these structures are also bulky, and require great care in their design and manufacture in order to make them successful for many electrical and electronic applications.
To provide a pleasing tactile xe2x80x9csnapxe2x80x9d to the above constructions, a layer of formed metal foil shapes may also be applied to replace the shunt layer. These shapes are typically convex on their outer surface and concave on their interior surface. By placing the formed metal foil shapes above the isolating mask layer opening, opposite a switch contact pair, applied mechanical pressure causes the shapes to temporarily invert, thus making contact between the switch contacts. This method allows both signal and power electrical charges to be passed between switch pairs. As this construction also requires individual layers to be assembled, including illuminated actuating elastomeric structures and frames, a bulky and complex assembly results.
Application of electroluminescent lamp as an illumination scheme to the above methodologies provides a thinner structure, however there are still numerous individual layers and actuators to be applied and aligned to complete an illuminated membrane switch assembly. An example of this process is referenced in U.S. Pat. No. 5,680,160 (the ""160 patent), wherein LaPointe describes such an application consisting of screen-printed illumination and electrical contacts arranged in a pattern such as might be used for a map as a teaching tool in geography. However, this method only provides illumination during switch contact, and is also limited in the amount of electrical current the switch contacts may carry. The use of conductive inks as switch elements also severely limits their useful life cycle. Additionally, this method does not provide electrical circuit separation between the switch portion and the illumination circuit portion without introducing an additional switch contact and shunt set with attendant construction and isolation layers. Thus, high voltage alternating current may add electrical interference to the switch circuit. As the switch circuit may also make contact for voltage sensitive semiconductor devices, this lack of isolating circuits may cause both electrical interference to, and failure of such devices.
In U.S. Pat. No. 5,667,417, Stevenson teaches a method of producing low cost metal foil based electroluminescent lamps of potentially complex graphic pattern by using a precise indexing system that applies well known flexible circuit technology to a cost-effective continuous production process. Application of this process to the manufacture of illuminated membrane switches can result in switch assemblies that are both low-cost, plus electrically and mechanically superior to those described in the ""160 patent.
Thus, there is a need for low profile illuminated membrane switch assemblies that provide all the elements of individually addressable illuminated areas, electrically separated switch and illumination circuitry, plus robust current carrying switch contacts and shunting means. Further, there is a need to provide such a low profile membrane switch assembly that may be made from a single flexible substrate material applied to an automated manufacturing system.
The present invention is directed to a method of manufacturing EL illuminated membrane switches incorporating some of the processes used in the manufacture of flexible printed circuit boards.
In an exemplary embodiment of the invention, the method of the present invention includes the following steps. In the first step, a light transmissive process carrier film having metal foil bonded to its surface is prepared for further process by die cutting or chemically etching the bonded metal foil to from the desired front capacitive electrode bus, membrane switch contacts and electrical shunt, power input distribution elements and associated electrical contacts to produce a planar flexible circuit board. Following this, the basis flexible circuit board carrier film is placed onto a commercially available transport system that incorporates an optical registration system to precisely position the image area for the remaining print and die cutting process cycles. This method allows the precise (+/xe2x88x92 less than 0.002xe2x80x3 in X, Y and xcex8 axis) physical positioning of the basis carrier film without deleterious effect upon the positioning reference means. Using this positioning method allows practically unlimited numbers of print layers to be applied, and final die cutting of the completed product, without concern for layer-to-layer alignment.
The third step consists of printing a light transmissive, electrically conductive ink to precisely form a capacitive front electrode. Through precise, optically registered positioning the capacitive front electrode ink is allowed minimal bleed onto the front capacitive electrode bus.
In the fourth step a high dielectric, hygrophobically compounded EL phosphor ink is printed over the front electrode ink to further define the illuminated area. Precise, optically registered positioning of the basis carrier film allows precision phosphor application onto the front capacitive electrode element. Following this, in the fifth step, a layer of capacitive dielectric ink is applied to cover the EL phosphor layer, completely isolating the front capacitive electrode, phosphor layers and their associated power distribution elements. The capacitive dielectric layer ink is allowed to bleed beyond the EL phosphor layer and front electrode elements and power distribution elements to provide this electrical isolation.
Next then, in step six, a rear electrode layer of electrically conductive ink is applied to further define the precise illuminated area. This layer is allowed to bleed onto the rear electrode power distribution element, providing an electrical path to input power.
In step seven; a polyester film or ultraviolet activated dielectric coating is applied to the entire metal foil surface of the process carrier film. Openings in this layer are made allowing exposure of the metal foil layer to precisely define membrane switch contacts and electrical shunt, plus isolated electrical power contact termination areas.
Steps eight and nine comprise the printing of an isolation element and an actuating element from thick film elastomeric ink. The isolation element is printed as a frame shape surrounding the shunt portion, while the actuating element is printed as a hemispherical bump on top of the dielectric coating and is centered over the EL rear electrode.
Following this step, the completed EL lamp and membrane switch subassembly is then cut from the basis carrier film, then folded into three layers comprising the switch contact layer, the shunt layer and the illuminated actuator layer to which mechanical force may be applied to operate the switch.
A first embodiment of an EL illuminated membrane switch manufactured by the method of the present invention comprises a light transmissive, single-sided flexible printed circuit substrate containing both switch and EL lamp elements, electrical distribution elements and electrical input and output terminations. The EL lamp layers are progressively applied beginning with the front electrode light transmissive, electrically conductive ink, followed by hygrophobically compounded electroluminescent phosphor ink to define the illumination pattern, then capacitive dielectric ink to electrically isolate the front electrode and phosphor layers, followed by an electrically conductive ink layer that defines the rear capacitive electrode, finishing with an electrically insulated and environmentally isolated encapsulation layer that is patterned to protectively insulate all EL portions while leaving exposed all switch elements and electrical contacts. Flexible, thick-film elastomeric ink is then applied to create both a switch isolation mask pattern located around the switch shunt portion and a mechanical actuator bump on the rear surface of the EL lamp portion. The EL illuminated membrane switch is then die-cut from the surrounding substrate material, folded into three layers that comprise switch, shunt and illuminated portions to complete the assembly.
In a second preferred embodiment, a double-sided flexible circuit substrate with switch contacts and switch shunt, associated electrical distribution elements and electrical contact terminals formed on one surface; EL lamp rear electrode and front capacitive electrode bus elements, electrical distribution elements and electrical input contact terminals are formed upon the opposite surface. EL lamp layers are sequentially applied in order of a first capacitive dielectric layer isolating the rear electrodes and associated electrical distribution elements from the front electrode bus; application of hygrophobically compounded electroluminescent phosphor ink on top of the capacitive dielectric layer to precisely define the illuminated pattern; application of electrically conductive, light transmissive ink over the EL phosphor layer and bridging onto the front capacitive electrode power distribution bus to create a front capacitive electrode; then, application of a light transmissive, electrically insulated and environmentally isolated encapsulation layer that is patterned to protectively insulate all EL portions while leaving exposed all EL lamp portion electrical contacts. The EL illuminated membrane switch subassembly is then die-cut and formed from the surrounding substrate material, creating an embossed portion surrounding the switch shunt acting as a spring element, thus isolating the shunt; then folded into three layers that comprise switch, shunt and illuminated portions to complete the assembly.
In a third preferred embodiment, a double-sided flexible circuit substrate with switch contacts and switch shunt, (the shunt element positioned approximately opposite the EL lamp rear capacitive electrode center), electrical distribution elements and electrical contacts formed on one surface; EL lamp rear capacitive electrode and front capacitive electrode power distribution bus elements, electrical distribution elements and electrical input contact terminations are formed upon the opposite surface. EL lamp layers are sequentially applied in order of first capacitive dielectric layer to isolate the rear capacitive electrodes and their associated electrical distribution elements from the front capacitive electrode bus; application of hygrophobically compounded electroluminescent phosphor ink on top of the capacitive dielectric layer to precisely define the illuminated pattern; application of electrically conductive, light transmissive ink over the EL phosphor layer bleeding onto the front capacitive electrode power distribution bus to create a front capacitive electrode; then application of a light transmissive, electrically insulated and environmentally isolated encapsulation layer that is patterned to protectively insulate all EL portions leaving exposed all EL lamp portion electrical contact terminals. The EL illuminated membrane switch is then die-cut and formed from the surrounding substrate material, creating an embossed portion that acts as a spring element surrounding an aperture opening isolating the shunt from the switch contacts; finally then, folded into three layers that comprise switch portion, isolation layer portion, shunt and illuminated portion to complete the assembly.
The method of the present invention provides the ability to manufacture EL illuminated membrane switches at a cost fractional of that of comparable conventional construction. Additionally, these lower-cost EL illuminated membrane switches can be manufactured on readily obtainable automated production equipment. Further features and advantages of the present invention will be appreciated by a review of the following detailed description when taken in conjunction with the following drawings.